Abstract/Summary Nitrate is the most commonly found contaminant in the world's aquifers and a major contaminant of drinking water in the U.S. Nitrate causes fetal growth restriction (FGR), low birth weight (LBW), and birth defects (BD) in studies of experimental animals, but only at high concentrations. High levels of nitrate in drinking water are also a well-recognized cause of a potentially fatal form of anemia (methemoglobinemia or blue baby syndrome) in newborns. However, surprisingly few epidemiologic studies have examined the risk of adverse birth outcomes from contemporary lower levels of nitrate in water. There is suggestive evidence from these studies that nitrate in drinking water may increase the risk of FGR, preterm delivery (PTD) and neural tube, limb deficiencies and cleft palate/lip BD. Most of the prior studies are limited by their ecologic design, small size, uncertain estimates of exposure, poor control of other risk factors, and lack of consideration of dietary intake of nitrate and nitrite. There is a clear need for larger and better studies at contemporary levels of exposure, which this proposal is designed to address. The goal of this proposal is to evaluate whether maternal consumption of nitrate contaminated water increases the risk of adverse birth outcomes. Specifically we aim to examine whether nitrate in drinking water is associated with an increased risk of: 1) PTD (< 37 weeks) and very PTD (<32 weeks), 2) FGR based on being small (<10th percentile) for gestational age, LBW (<2.5 kg among infants born at term), reduced birth head circumference and body length, 3) neural tube, limb deficiencies and cleft palate/lip BD, and 4) interacts with the use of nitrosatable drugs for BD, which has been reported in one study. To address these aims we will capitalize on unique resources available in Denmark, which permit estimation of concentrations of nitrate in drinking water for all addresses and linkage of these estimates with complete address histories, data on birth outcomes, prescription drug use and other risk factors (e.g. maternal smoking, parity, and socio-economic status) from Danish registries to create a population based cohort study of all liveborn singleton births in Denmark from 1997 to 2013 (~1 million births). We will also link these data with the Danish National Birth Cohort (DNBC), which has extensive information on diet (for ~67000 births) which will be used for estimating dietary intake of nitrate and nitrite. The DNBC also has useful data on water consumption, drug use (prescription and non-prescription), and extensive information on other risk factors. Our study will be the largest ever conducted, with comprehensive data on nitrate intake from water and food and on other risk factors. Our findings should be highly influential in the development of policies and recommendations concerning nitrate in drinking water, which would have a very large public health impact given the ubiquity of nitrate in drinking water and the severity of these outcomes.